Anterior Shoulder Dislocation: Most Common Nerve Injuries, Symptoms, and Management

What is the most common nerve injured in anterior shoulder dislocation?

The most common nerve injured in an anterior shoulder dislocation is the axillary nerve, primarily due to its close anatomical proximity to the humeral head and glenohumeral joint capsule, making it highly susceptible to stretch or compression during the forceful displacement of the humerus.

Understanding Anterior Shoulder Dislocation

Anterior shoulder dislocation occurs when the head of the humerus (upper arm bone) is forcibly displaced forward and downward out of the glenoid fossa (socket) of the scapula (shoulder blade). This is the most prevalent type of shoulder dislocation, often resulting from trauma such as a fall onto an outstretched arm, a direct blow to the shoulder, or sports-related injuries.

The mechanism typically involves excessive external rotation and abduction of the arm. When the humeral head dislocates anteriorly, it can put significant stress on surrounding neurovascular structures, leading to potential damage.

The Axillary Nerve: Anatomy and Vulnerability

The axillary nerve (C5-C6) is a branch of the posterior cord of the brachial plexus. Its anatomical course makes it particularly vulnerable during anterior shoulder dislocation:

• Origin and Course: It originates in the axilla, then passes posteriorly through the quadrangular space (bounded by the teres major, teres minor, long head of triceps, and surgical neck of the humerus). It then wraps around the surgical neck of the humerus, lying in direct contact with the bone.
• Innervation:
  • Motor: It primarily innervates the deltoid muscle, which is responsible for the majority of shoulder abduction (lifting the arm away from the body), and the teres minor muscle, a rotator cuff muscle that assists in external rotation of the arm.
  • Sensory: It provides sensory innervation to the skin over the lateral aspect of the shoulder, often referred to as the "regimental badge" area.
• Why it's Vulnerable: As the humeral head dislocates anteriorly, it can stretch, compress, or even avulse the axillary nerve due to its intimate relationship with the surgical neck of the humerus. The nerve can be directly impaled, compressed by the dislocated humeral head, or stretched excessively as the bone moves out of the joint.

Clinical Manifestations of Axillary Nerve Injury

Injury to the axillary nerve can present with a combination of motor and sensory deficits:

• Motor Deficits:
  • Weakness or paralysis of the deltoid muscle: This is the most prominent sign, leading to difficulty or inability to abduct the arm beyond approximately 15 degrees (the initial abduction is often performed by the supraspinatus).
  • Weakness in external rotation: Due to impaired function of the teres minor.
  • Shoulder atrophy: Chronic injury can lead to visible wasting of the deltoid muscle, making the shoulder appear flattened.
• Sensory Deficits:
  • Numbness, tingling, or altered sensation: Over the lateral aspect of the shoulder (the "regimental badge" area).
• Pain: While not always present, nerve injury can cause neuropathic pain.

Diagnosis of axillary nerve injury typically involves a thorough clinical examination, including assessment of motor strength and sensory perception. Electromyography (EMG) and nerve conduction studies (NCS) may be performed to confirm the diagnosis, localize the injury, and assess its severity.

Other Nerves Potentially at Risk (Less Common)

While the axillary nerve is the most frequently injured, severe or complex shoulder dislocations can, in rare cases, affect other nerves of the brachial plexus:

• Musculocutaneous Nerve: This nerve (C5-C7) innervates the biceps brachii, brachialis, and coracobrachialis muscles (responsible for elbow flexion and forearm supination) and provides sensation to the lateral forearm. Injury is uncommon but possible with extensive trauma.
• Radial Nerve: The radial nerve (C5-T1) is responsible for extension of the elbow, wrist, and fingers, and sensation over the posterior arm, forearm, and hand. While more commonly associated with humeral shaft fractures, severe dislocations can occasionally impact it.
• Median and Ulnar Nerves: These nerves (C5-T1 and C8-T1, respectively) primarily control muscles and sensation in the forearm and hand. Injury to these nerves in the context of a shoulder dislocation is rare and usually indicative of a more severe, widespread brachial plexus injury.

Management and Prognosis

Immediate management of an anterior shoulder dislocation involves prompt reduction (repositioning) of the humeral head back into the glenoid fossa. Following reduction, the focus shifts to assessing for associated injuries, including nerve damage.

• Observation: Many axillary nerve injuries resulting from dislocation are neuropraxias, meaning the nerve is stretched or compressed but not severed. These often resolve spontaneously within weeks to months as the myelin sheath recovers.
• Physical Therapy: Rehabilitation is crucial to restore range of motion, strength, and proprioception in the shoulder.
• Surgical Intervention: In cases of severe nerve injury (axonotmesis or neurotmesis), where there is no sign of recovery after several months, surgical exploration, neurolysis (freeing the nerve from scar tissue), or nerve grafting may be considered.

The prognosis for axillary nerve injury varies depending on the severity of the damage. Neuropraxia generally has an excellent prognosis for full recovery, while more severe injuries may result in persistent weakness or sensory deficits.

Conclusion

The axillary nerve stands out as the most common nerve injured in anterior shoulder dislocation due to its intimate anatomical relationship with the humeral head. Understanding this vulnerability is crucial for healthcare professionals and fitness educators alike. Prompt medical evaluation of any shoulder dislocation is paramount, not only for proper joint reduction but also for a comprehensive neurovascular assessment to identify and manage potential nerve injuries, thereby optimizing recovery and preventing long-term deficits.